LASER [Light Amplification by Stimulated Emission of Radiation]

Dr.Navjyot Trivedi, M.P.TSSIP, Bhavnagar

HISTORY

• Albert Einstein – 1st described this theory that was transformed in to laser therapy.

• By the end of the 60’s, Endre Mester (Hungary) -– was reporting on wound healing through laser therapy

• In early 1960’s, the 1st low level laser was developed.

• In Feb. 2002, the MicroLight 830 (ML830) received FDA approval for Carpal Tunnel Syndrome Treatment (research treatment)

• Laser therapy – has been studied in Europe for past 25-30 years; US 15-20 years

PROPERTIES OF LASER

1. MONOCHROMATICITY Only one wavelength with a defined frequency. When a laser light entering a prism would be IDENTICAL on exit,

because of MONOCHROMATICITY. 2. COHERENCE That is all the PEAKS & TROUGHS of the electrical & magnetic fields

occurs at the same time (TEMPORAL COHERENCE) & travel in the same direction (SPATIAL COHERENCE).

3. NON DIVERGENCE It refers to the relative parallelism of the laser beam. The more parallel the laser beam the greater the concentration of energy in

a localized area. The laser light has very low divergence property, so it does not tend to

spread out.

• Spontaneous emission is the process by which a quantum system such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to a state with a lower energy (e.g., the ground state) and emits quanta of energy.

• Light or luminescence from an atom is a fundamental process that plays an essential role in many phenomena in nature and forms the basis of many applications, such as fluorescent tubes, older television screens (cathode ray tubes), plasma display panels, lasers, and light emitting diodes.

• Lasers start by spontaneous emission, and then normal continuous operation works by stimulated emission.

• Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level.

• The liberated energy transfers to the electromagnetic field, creating a new photon with identical phase, frequency, polarization, and direction of travel as the photons of the incident wave.

• This is in contrast to spontaneous emission which occurs at random intervals without regard to the ambient electromagnetic field.

PRINCIPLE OF PRODUCTION OF LASER

• According to Quantum theory – electrons can only occupy certain energy levels or shells around the nucleus. (Stable / Ground / Lower State)

• If the atom is given additional energy say by heating, these outer electrons can be made to occupy higher energy levels. (Excited / Unstable state)

• Spontaneous emission occurs. The excited state returns to a lower energy state, emitting the excess energy as a photon, or quanta of light.

• Since there are many more of electrons, there is further stimulated emission of photons causing a CASCADE / AVALANCHE EFFECT. [(i.e.) One photon releasing another identical one photon, then these two stimulating two more & so on]

• Population Inversion – Having more atoms in the upper energy state than the lower energy state.

PRODUCTION – HELIUM-NEON LASER

• It consists of a cylindrical tube containing these gases at low pressure& surrounded by a flash gun / electrical supply.

• Inside the tube it is polished for the reflection to occur (One end of the tube fully polished & the other end partially polished) & so the molecules reverberate the walls in a highly agitated state, building energy.

• When the critical level is reached, the flow of energy literally BURSTS through the partially polished front end of the tube giving out the laser beam which is channeled along an optic fiber to the beam applicator or probe for treatment.

• The helium – neon lasers give radiation in the visible red region at 632.8nm.

TYPES OF LASERS

LASER TYPES WAVELENGTH RADIATION

RUBY 694.3nm Red Light

HELIUM - NEON 632.8nm Red Light

DIODE S 650 – 904nm Red Light - Infrared

The laser commonly used in physiotherapy is1. Helium – neon laser2. Gallium Aluminium Arsenide ( GaAlAs) - 810 – 850nm

– At near infrared wavelength

DEPTH OF PENETRATION1. Helium – Neon Laser – up to EPIDERMIS2. GaAlAs – up to EPIDERMIS & DERMIS

CLASSIFICATION OF LASERS

RANGE OF POWER USAGE EFFECTS

LOW POWER 1. Blackboard pointer2. Supermarket barcode reader

No effect on Eye or skin

MID POWER Models up to 50mW mean power(3A – 3B, LLLT) , Used therapeutically for the physiotherapy treatment

Safe on skin & Not in eye

HIGH POWER Surgical Destructive Not safe on skin & eye

INTENSIY OF LASERThe intensity of laser used in physiotherapy treatment range from

1mWcm¯² to 50mWcm¯².It is relatively very low power & Intensity.The beam diameter is about 3mm which is used clinically.PHYSICAL EFFECTS OF LASER1. Heat Production – Reversible process2. Dehydration – Reversible process3. Coagulation of proteins, Thermolysis & Evaporation SHOULD NOT

OCCUR with the laser dosage.Effects of laser on the tissues

Reflected Absorbed Transmitted (Penetration)

PHYSIOLOGICAL EFFECTS OF LASER• Thermal effect – produce Heating in the tissues – Increased cell wall

permeability.• Initiate chemical change,• Disrupt molecular bonds & produce free radicals

THERAPEUTIC EFFECTS OF LASER• Tissue / Wound Healing• Additional factors involved are – Increase in Collagen formation,

Vasodilatation & DNA Synthesis.• Pain Reduction – Both Acute & Chronic• Painful Soft tissue Injuries ( of swelling) Oedema

• Musculoskeletal Conditions• Swelling of Joints• Neurogenic pain states• Myofascial pain• Post traumatic joint disorders & Burns(Indications - Facilitate wound healing, Pain reduction, Increasing the

tensile strength of a scar, Decreasing scar tissue, Decreasing inflammation, Bone healing and fracture consolidation)

INDICATIONS OF LASER

CONTRAINDICATION OF LASER1. Cancer Tissue2. Pregnant Uterus3. Hemorrhage4. Infected tissue5. Epileptic 6. Cardiac patients7. PacemakersDANGER & PRECAUTIONS1. Eyes2. Avoid reflecting the laser beam from shiny surfaces3. Only switch ON the laser when the applicator is in CONTACT

with the skin4. Use the appropriate protective goggles.

GENERAL POINTS TO NOTE

1. Treatment is applied to the skin by a HAND – HELD APPARATUS.2. Direct application to the skin ensures MAXIMUM transfer of laser

energy.3. Need to wear the GOGGLES4. The treatment surface is cleaned with ALCOHOL.5. It is important to maintain the laser applicator in contact with the tissues.6. The laser beam is applied at RIGHT ANGLES.7. The applicator contact should not provoke TENDERNESS / PAIN.8. Visible red light laser (HeNe) is recommended for SUPERFICIAL

CONDITIONS – Wounds, Ulcers & Skin conditions.9. Infrared laser is for DEEPER MUSCULOSKELETAL STRUCTURES

(GaAlAs)10. Low power (BioStimulative) lasers provide NO SENSATION to the

patient.

LASER PARAMETERS1. Type of Laser – HeNe, GaAlAs2. Wavelength of Laser3. Power of Laser4. Size of the treatment area5. Exposure timeENERGY DENSITY/RADIANT EXPOSURE1. The amount of energy delivered to the patient is measured in JOULES

PER CENTIMETER SQUARE. J/Cm².It can be calculated by a formula consisting of ( The time of exposure,

The power of the laser & the size of the area)

EXAMPLE FOR CALCULATING DENSITYMean power = 10mw, Treatment Area = 0.125cm², Time of exposure = 50sec10mw/0.125cm² = 80mw/cm², Energy Density = 80mw/cm² x 50sec, =

4000mJ/cm²= 4J/cm²

The usual ranges are from 1 to 10J/cm²Dose as low as 0.5 J/cm²Dose as high up to 32 J/cm²The therapeutic laser range from 0.5J/cm² & 4J/cm²It is generally recommended that the LOW PULSE FREQUENCIES & LONG

PULSE DURATIONS – ACUTE CONDITION.HIGHER PULSE FREQUENCY & SHORT PULSE DURATION –

CHRONIC CONDITIONSNote:-1. Low power laser DOES NOT PRODUCE THERMAL RESPONSE.2. The power of laser is preset within the device along with the wavelength.

Table - Suggested Treatment ApplicationsApplication Laser Type Energy Density

Trigger point      Superficial HeNe 1-3 J/cm2

  Deep GaAs 1-2 J/cm2

Edema reduction      Acute GaAs 0.1-0.2 J/cm2

  Subacute GaAs 0.2-0.5 J/cm2

Wound healing (superficial tissues)    

  Acute HeNe 0.5-1 J/cm2

  Chronic HeNe 4 J/cm2

Wound healing (deep tissues)    

  Acute GaAs 0.05-0.1 J/cm2

  Chronic GaAs 0.5-1 J/cm2

Scar tissue GaAs 0.5-1 J/cm2

Copied with permission from Physio Technology.

Table - Parameters of Low-Output Lasers

Helium Neon (HeNe) Gallium Arsenide (GaAs)Laser type Gas SemiconductorWavelength 632.8 nm 904 nmPulse rate Continuous wave 1-1000 HzPulse width Continuous wave 200 nsecPeak power 3 mW 2 WAverage power 1.0 mW 0.04-0.4 mWBeam area 0.01 cm 0.07 cmFDA class Class II laser Class I laser

• LASER TREATMENT TECHNIQUES• The method of application of laser therapy is relatively simple, but certain principles of dosimetry should

be discussed so the clinician can accurately determine the amount of laser energy delivered to the tissues. For general application, only the treatment time and the pulse rate vary. For research purposes, the investigator should measure the exact energy density emitted from the applicator before the treatments. Dosage is the most important variable in laser therapy and may be difficult to determine because of the variables mentioned previously (e.g., hours of operation or condition of the unit).

• Lasing Techniques• · Gridding• · Scanning• · Wanding• The laser energy is emitted from a handheld remote applicator. The GaAs laser houses the semiconductor

elements in the tip of the applicator, whereas the HeNe lasers contain their componentry inside the unit and deliver the laser light to the target area via a fiber-optic tube. The fiber-optic assembly is fragile and should not be crimped or twisted excessively. The fiber-optics used with the HeNe and the elliptical shape of the GaAs laser create beam divergence with both devices. This divergence causes the beam's energy to spread out over a given area so that as the distance from the source increases, the intensity of the beam lessens.

• LASING TECHNIQUES• To administer a laser treatment, the tip should be in light contact with the skin and directed

perpendicularly to the target tissue while the laser is engaged for the designated time. Commonly, a treatment area is divided into a grid of square centimeters, with each square centimeter stimulated for the specified time.

• This gridding technique is the most frequently utilized method of application and should be used whenever possible. Lines and points should not be drawn on the patient's skin, because this may absorb some of the light energy. If open areas are to be treated, a sterilized clear plastic sheet can be placed over the wound to allow surface contact.

• An alternative is a scanning technique in which there is no contact between the laser tip and the skin. With this technique, the applicator tip should be held 5-10 mm from the wound.

• Because beam divergence occurs, there is a decrease in the amount of energy as the distance from the target increases. The amount of energy lost becomes difficult to quantify accurately if the distance from the target is variable. Therefore, it is not recommended to treat at distances greater than 1 cm. When using a laser tip of 1 mm with 30 degrees of divergence, the red laser beam of the HeNe should fill an area the size of 1 cm2. Although the infrared laser is invisible, the same consideration should be given when using the scanning technique. If the laser tip comes into contact with an open wound, the tip should be cleaned thoroughly with a small amount of bleach or other antiseptic agents to prevent cross-contamination.

• Treatment Tip•  • In treating myofascial trigger points, the therapist should use a gridding

laser technique with the probe held perpendicular to the skin with light contact. The energy density should be set at 3 J/cm2. The laser treatment can be combined with electrical stimulation using low-frequency (1-5 Hz) high-intensity current to produce pain modulation via the release of b-endorphin.

• The scanning technique should be differentiated from the wanding technique, in which a grid area is bathed with the laser in an oscillating fashion for the designated time. As in the scanning technique, the dosimetry is difficult to calculate if a distance of less than 1 cm cannot be maintained. The wanding technique is not recommended because of irregularities in the dosages.

• DOSAGE• The HeNe laser has a 1.0-mW average power output at the fiber tip and is delivered in the continuous wave mode. The GaAs laser has an output of 2 W but

has only a 0.4-mW average power when pulsed at its maximum rate of 1000 Hz. The frequency of the GaAs is variable, and the clinician may choose a pulse rate of 1-1000 Hz, each with a pulse width of 200 nsec (nsec = 10-9) and.

• The pulsed modes drastically reduce the amount of energy emitted from the laser. For example, a 2-W laser is pulsed at 100 Hz:•  • Average power= pulse rate ´ peak power ´ pulse width= 100 Hz ´ 2 W ´ (2 ´ 10-7 sec)= 0.04 mW•  • This contrasts with the power output of 0.4-mW with the 1000 Hz rate. Therefore, it can be seen that adjustment of the pulse rate alters the average power,

which significantly affects the treatment time if a specified amount of energy is required. In the past it was thought that altering the frequency of the laser would increase its benefits. Recent evidence indicates that the total number of Joules is more important; therefore, higher pulse rates are recommended to decrease the treatment time required for each stimulation point

• The dosage or energy density of laser is reported in the literature as joules per square centimeter (J/cm2). One joule is equal to 1 W/sec. Therefore, dosage is dependent on (1) the output of the laser in mW, (2) the time of exposure in seconds, and (3) the beam surface area of the laser in cm2.

• Dosage should be accurately calculated to standardize treatments and to establish treatment guidelines for specific injuries. The intention is to deliver a specific number of J/cm2 or mJ/cm2. After setting the pulse rate, which determines the average power of the laser, only the treatment time per cm2 needs to be calculated.

•  • TA= (E/Pav) ´ ATA= treatment time for a given areaE= mJ of energy per cm2Pav= Average laser power in mWA= beam area in cm2

•  • For example: To deliver 1 J/cm2 with a 0.4 mW average-power GaAs laser with a 0.07 cm2 beam area:•  • TA= (1 J/cm2/0.0004 W) ´ 0.07 cm2= 175 sec or 2:55 min•  • To deliver 50 mJ/cm2 with the same laser, it would only take 8.75 seconds of stimulation. Charts are available to assist the clinician in calculating the

treatment times for a variety of pulse rates. The GaAs laser can only pulse up to 1000 Hz, resulting in an average energy of 0.4 mW. Therefore, the treatment times may be exceedingly long to deliver the same energy density with a continuous wave laser.

• DEPTH OF PENETRATION• Any energy applied to the body can be absorbed, reflected, transmitted, and refracted.

Biologic effects result only from the absorption of energy, and as more energy is absorbed, there is less available for the deeper and adjacent tissues.

• Laser light's depth of penetration depends on the type of laser energy delivered. Absorption of HeNe laser energy occurs rapidly in the superficial structures, especially within the first 2-5 mm of soft tissue. The response that occurs from absorption is termed the "direct effect." The "indirect effect" is a lessened response that occurs deeper in the tissues. The normal metabolic processes in the deeper tissues are catalyzed from the energy absorption in the superficial structures to produce the indirect effect. HeNe laser has an indirect effect on tissues up to 8-10 mm.

• The GaAs, which has a longer wavelength, is directly absorbed in tissues at depths of 1-2 cm and has an indirect effect up to 5 cm. Therefore, this laser has better potential for the treatment of deeper soft tissue injuries, such as strains, sprains, and contusions. The radius of the energy field expands as the nonabsorbed light is reflected, refracted, and transmitted to adjacent cells as the energy penetrates. The clinician should stimulate each square centimeter of a "grid," although there will be an overlap of areas receiving indirect exposure.

• CLINICAL APPLICATIONS FOR LASERS• Because the production of lasers is relatively new, the biologic and physiologic effects

of this concentrated light energy are still being explored. The effects of low-power lasers are subtle, primarily occurring at a cellular level. Various in vitro and animal studies have attempted to elucidate the interaction of photons with the biologic structures. Although there are few controlled clinical studies in the literature, documented case studies and empirical evidence indicate that lasers are effective in reducing pain and aiding wound healing. The exact mechanisms for action are still uncertain, although proposed physiologic effects include an acceleration in collagen synthesis, a decrease in microorganisms, an increase in vascularization, reduction of pain, and an anti-inflammatory action.

• Low-level lasers are best recognized for increasing the rate of wound and ulcer healing by enhancing cellular metabolism. Results from animal studies have varied as to the benefits on wound healing, perhaps owing to the fact that the types of lasers, dosages, and protocols used have been inconsistent. In humans, improvement of nonhealing wounds indicates promising possibilities for treatment with lasers.

• WOUND HEALING• Early investigations of the effects of low-power laser on biologic tissues were limited to in vitro experimentation. Although it was known that high-power lasers

could damage and vaporize tissues, little was known about the effect of small dosages on the viability and stability of cellular structures. It was found that low dosages (<10 J/cm2) of radiation from low-level lasers had a stimulatory action on metabolic processes and cell proliferation compared to incandescent or tungsten light.

• Mester conducted numerous in vitro experiments with two lasers in the red portion of the visual spectrum: the ruby laser, wavelength of 694.3 nm, and the HeNe laser, wavelength 632.8 nm. Human tissue cultures showed significant increases in fibroblastic proliferation following stimulation by either laser tested. Fibroblasts are the precursor cells to connective tissue structures such as collagen, epithelial cells, and chondrocytes. When the production of fibroblasts is stimulated, one should expect a subsequent increase in the production of connective tissue. Abergel and associates documented that certain dosages of HeNe and GaAs laser, wavelength 904 nm, caused in vitro human skin fibroblasts to have a threefold increase in procollagen production. This effect was most marked when low-level stimulation (1.94 ´ 10-7 to 5.84 ´ 10-6 J/cm2 of GaAs and dosages of 0.053 to 1.589 J/cm2 of HeNe) was repeated over 3-4 days versus a single exposure. Samples of tissue showed increases in fibroblast and collagenous structures as well as increases in the intracellular material and swollen mitochondria of cells. Furthermore, cells were undamaged in regard to their morphology and structure after exposure to low-power lasers.

• Analysis of the cellular metabolism, with attention to the activity of DNA and RNA, has been made.Through radioactive markers, it was suggested that laser stimulation enhances the synthesis of nucleic acids and cell division. Abergel reported that laser-treated cells had significantly greater amounts of procollagen messenger RNA, further confirming that increased collagen production occurs because of modifications at the transcriptional level.

• Low-level lasers were used in animal studies to further delineate both the beneficial applications of laser light and its potential harm. In an early study by Mester and associates, mechanical and burn wounds were made on the backs of mice. Similar wounds on the same animals served as the controls, with the experimental wounds subjected to various doses of ruby laser. Although there were no histologic differences among the wounds, the lased wounds healed significantly faster, especially at a dosage of 1 J/cm2. It was also demonstrated that repeated laser treatments were more effective than a single exposure.

• Other researchers investigated the rate of healing and tensile strength of full-thickness wounds when exposed to laser irradiation. There were conflicting reports regarding rates of healing, with some studies showing no change in the rate of wound closure, and others showing significantly faster wound healing. Although the experimental results were conflicting, an explanation for the discrepancy may be an indirect systemic effect of laser energy. Mester showed that it was not necessary to irradiate an entire wound to achieve beneficial results, because stimulation of remote areas had similar results. Kana and associates described an increase in the rate of healing of both the irradiated and nonirradiated wounds on the same animal compared to nonirradiated animals. This systemic effect was most marked with the argon laser. Several studies that investigated the rate of healing on living animal tissue used a second, nontreated control wound on the same animal. The rate of healing may have been confounded by this systemic effect. Whether the systemic effect involves a humoral component, a circulating element, producing immunologic effects has yet to be determined or identified. Bactericidal and lymphocyte stimulation are proposed mechanisms for this phenomenon.

• Tensile Strength• The increased tensile strength of lased wounds was confirmed more often.

Wound contraction, collagen synthesis, and increases in tensile strength are fibroblast-mediated functions and were demonstrated most markedly in the early phase of wound healing. Wounds were tested at various stages of healing to determine their breaking point, and were compared to a control or nonlased wound. Laser-treated wounds had significantly greater tensile strengths, most commonly in the first 10-14 days after injury, although they approached the values of the control after that time. Hypertrophic scars did not result as tissue responses normalized after a 14-day period. HeNe laser of doses ranging from 1.1 to 2.2 J/cm2 elicited positive results when lased either twice a day or on alternate days. The increased tensile strength corresponds to higher levels of collagen.

• Immunologic Responses• These early studies led to the hypothesis that laser exposure could enhance healing of skin and connective tissue lesions, but the

mechanism was still unclear. Biochemical analysis and radioactive tracers were used to delineate the immunologic effects of laser light on human tissue cultures. The laser irradiation caused increased phagocytosis by leukocytes with dosages of 0.05 J/cm2. This led to the possibility of a bactericidal effect, which was further demonstrated with laser exposures on cell cultures containing Escherichia coli, a common intestinal bacteria in humans. The ruby laser had an increased effect both on cell replication and on the destruction of bacteria via the phagocytosis of leukocytes. Mester also concluded that there were immunologic effects with the ruby, HeNe, and argon lasers. Specifically, there was a direct stimulatory influence on the T- and B-lymphocyte activity, a phenomenon that is specific to laser output and wavelength. HeNe and Argon lasers gave the best results, with dosages ranging from 0.5 to 1 J/cm2. Trelles did similar investigations in vitro and in vivo and reported that laser did not have bactericidal effects alone, but when used in conjunction with antibiotics, there were significantly higher bactericidal effects compared to controls.

• With the confidence that they would cause little or no harm and that they could serve a therapeutic purpose, low-power lasers have been used clinically on human subjects since the 1960s. In Hungary, Mester treated nonhealing ulcers that did not respond to traditional therapy with HeNe and argon lasers with respective wavelengths of 632.8 and 488 nm. The dosages were varied but had a maximum of 4 J/cm2. By the time of Mester's publication, 1125 patients had been treated, of which 875 healed, 160 improved, and 85 did not respond. The wounds, which were categorized by etiology, took an average of 12-16 weeks to heal. Trelles also showed promising results clinically using the infrared GaAs and HeNe lasers on the healing of ulcers, nonunion fractures, and on herpetic lesions.

• Gogia and associates, in the United States, treated nonhealing wounds with GaAs lasers pulsed at a frequency of 1000 Hz for 10 sec/cm2 with a sweeping technique held about 5 mm from the wound surface. This protocol was used in conjunction with daily or twice daily sterile whirlpool treatments and produced satisfactory results, although statistical information was not reported. Empirical evidence by these authors suggested faster healing and cleaner wounds when subjected to GaAs laser treatment three times per week.

• Inflammation• Biopsies of experimental wounds were examined for prostaglandin activity

to delineate the effect of laser stimulation on the inflammatory process. A decrease in prostaglandin (PGE2) is a proposed mechanism in which laser therapy promotes the reduction of edema. During inflammation, prostaglandins cause vasodilation, which contributes to the flow of plasma into the interstitial tissue. By reducing prostaglandins, the driving force behind edema production is reduced.8 The prostaglandin E and F contents were examined after treatments with HeNe laser at 1 J/cm2. In 4 days, both types of prostaglandins accumulated more than the controls. However, at 8 days, the PGE2 levels decreased, whereas PGF2 alpha increased. There was also an increased capillarization during this phase. Data indicate that prostaglandin production is affected by laser stimulation, and these changes possibly reflect an accelerated resolution of the acute inflammatory process.

• Scar Tissue• Macroscopic examination of healed wounds was subjectively described after the

laser experiments in most studies. In general, the wounds exposed to laser irradiation had less scar tissue and a better cosmetic appearance. Histologic examination showed greater epithelialization and less exudative material.

• Studies that utilized burn wounds showed more regular alignment of collagen and smaller scars. Trelles lased third-degree burns on the backs of hairless mice with GaAs and HeNe lasers and showed significantly faster healing in the lased animals. The best results were obtained with the GaAs laser because of its greater penetration. Trelles found increased circulation with the production of new blood vessels in the center of the wounds compared to the controls. Edges of the wounds maintained viability and contributed to the epithelialization and closure of the burn. Because there was less contracture associated with irradiated wounds, laser treatment has been suggested for burns and wounds on the hands and neck, where contractures and scarring can severely limit function.

• Clinical Considerations• There have been no ill effects reported from laser treatments for wound healing.

More controlled clinical data are needed to determine efficacy and to establish dosimetry that elicits reproducible responses. The impressions of low power lasers are that they have a biostimulative effect on impaired tissues unless higher dosages, in excess of 8-10 J/cm2, are administered. This effect does not influence normal tissue. Beyond these ranges a bioinhibitive effect may occur.

• The applications of the low-power laser in a clinical environment are potentially unlimited. Its applications can include wound healing properties on lacerations, abrasions, or infections. Clean procedures should be maintained to prevent cross contamination of the laser tip. Because the depth of penetration of the infrared laser is about 5 cm, other soft-tissue injuries can be treated effectively by laser irradiation. Sprains, strains, and contusions have been observed by the authors to have faster healing rates with less pain. Acupuncture and superficial nerve sites also can be lased or combined with electrical stimulation to treat painful conditions.

• PAIN• Lasers have also been effective in reducing pain and have been shown to affect peripheral nerve activity.

Rochkind and others produced crush injuries in rats and treated experimental animals with 10 J/cm2 of HeNe laser energy transcutaneously along the sciatic nerve projection. The amplitude of electrically stimulated action potentials was measured along the injured nerve and compared with controls up to 1 year later. The amplitude of the action potentials was 43 percent greater after 20 days, which was the duration of laser treatment. By 1 year, all lased nerves demonstrated equal or higher amplitudes than preinjury. The controls followed an expected course of recovery and did not reach normal levels even after 1 year.

• The effect of HeNe irradiation on peripheral sensory nerve latency has been investigated on humans by Snyder-Mackler and Bork. This double-blind study showed that exposure of the superficial radial nerve to low dosages of laser resulted in a significantly decreased sensory nerve conduction velocity, which may provide information about the pain-relieving mechanism of lasers. Other explanations for pain relief may be the result of hastened healing, anti-inflammatory action, autonomic nerve influence, and neurohumoral responses (serotonin, norepinephrine) from descending tract inhibition.

• Chronic pain has been treated with GaAs and HeNe lasers, and positive results have been observed empirically and through clinical research. Walker conducted a double-blind study to document analgesia after exposure to HeNe irradiation in chronic pain patients compared with sham treatments.When the superficial sites of the radial, median, and saphenous nerves as well as painful areas were exposed to laser irradiation, there were significant decreases in pain and less reliance on medication for pain control. These preliminary studies suggest positive results, although pain modulation is difficult to measure objectively.

• BONE RESPONSE• Future uses of laser irradiation include the treatment of other connective tissue structures,

such as bone and articular cartilage. Schultz and colleagues studied various intensities of Nd:YAG laser on the healing of partial-thickness articular cartilage lesions in guinea pigs. During the surgical procedure, the lesions were irradiated for 5 seconds, with intensities ranging from 25 to 125 J. After 4 weeks, the low-dosage group (25 J) had chondral proliferation, and by 6 weeks the defect had reconstituted to the level of the surface cartilage. Normal basophilia cells were present with staining, indicating normal cellular structures. The higher dosage groups and controls had little or no evidence of restoration of the lesion with cartilage. Bone healing and fracture consolidation have been investigated by Trelles and Mayayo. An adapter was attached to an intramuscular needle so that the laser energy could be directed deeper to the periosteum. Rabbit tibial fractures showed faster consolidation with HeNe treatment of 2.4 J/cm2 on alternate days. Histologic examination indicated more mature Haversian canals with detached osteocytes in the laser treated bone. There was also a remodeling of the articular line, which is impossible with traditional therapy. The use of lasers for the treatment of nonunion fractures has begun in Europe.

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